Solid-State Nuclear Magnetic Resonance Study Of Lithium/Sodium-Ion Battery Materials

Energy and environmental issues are important factors that constrain the development of science,technology and society.Under the background of China’s"carbon peaking and carbon neutrality" goals,it is of great significance to develop clean,efficient,and safe next-generation energy storage technologies.Lithium-ion batteries（LIBs）have received considerable attention in the field of portable mobile devices and power energy storage systems,while sodium-ion batteries are expected to be used in the next generation of large-scale energy storage due to their advantages such as abundant sodium resources and low cost.In addition,with the increasing demand for battery performance and safety performance,the development of high specific energy electrode materials and high safety of solid electrolyte materials has become the focus of research to optimize the energy density and safety performance of lithium/sodium ion batteries.Via the studies of ion transport mechanism,electrode reaction mechanism,structural evolution and interface characteristics in materials,we can deeply explore and understand the structure-property relationship of battery materials,so as to optimize the synthesis and modification methods of battery materials,as well as to promote the preparation of high-performance materials and the construction of efficient battery system.However,this exploration process is faced with various scientific difficulties.For example,the migration and disintercalation of lithium ions will cause the evolution of the complex local structure in the electrode material during the charging and discharging process.In addition,the products of the electrode/electrolyte interface layer are usually trace,disordered and complex,thus,it is difficult to effectively characterize and understand the electrode material from the microscopic level by traditional characterization methods such as diffraction.In contrast,nuclear magnetic resonance（NMR）technology has significant advantages in both studying the composition and micro local structure of materials,especially for the structure of amorphous materials.It is currently an extremely effective and non-destructive characterization tool in the research of lithium/sodium-ion batteries and solid-state batteries.In this thesis,we have conducted in-depth research on the ion transport mechanism,electrochemical reaction mechanism,and electrode/electrolyte interface issues of key materials in the next generation energy storage battery system through liquid NMR technology,high-resolution solid-state NMR technology,ex-situ electron paramagnetic resonance（EPR）technology,combined with ex-situ X-ray diffraction（XRD）,Raman and other technologies.For example,how does element doping affect the transport mechanism of lithium ions through regulating the local structure of the material to effect the conductive properties of the material for the sulfide solid electrolyte material Li₁₀SnP₂S_12-xSe_x（x=0.2,3,5,7,12）that can be applied to all solid-state batteries and solve the safety problems of batteries;Construction of a high interfacial stable sulfide solid electrolyte layer and its interfacial stabilization mechanism;The structure of a single ion conducting polymer electrolyte（NaPTAB-SGPE）and the mechanism behind its high cycle stability performance in sodium ion batteries;The sodium storage mechanism of SnO₂/C composite negative electrode materials for sodium ion batteries,as well as the formation and composition of the solid electrolyte interphase（SEI）layer of the electrode.（1）We have prepared a series of novel solid electrolyte materials for Li₁₀SnP₂S_12-xSe_x（x=0,2,3,5,7,12）by replacing the vertex S element in Li₁₀SnP₂S₁₂ with Se element with larger radius and less electronegativity.With the increase of x,the ionic conductivity of Li₁₀SnP₂S_12-xSe_x electrolytes firstly increases and then decreases,and has the highest ionic conductivity(6.67×10^-3 S/cm)at x=5,which is three times higher than that of Li₁₀SnP₂S₁₂.The effects of Se on the local structure regulation,Li⁺conductivity and Li⁺diffusion kinetics in Li₁₀SnP₂S_12-xSe_x electrolytes were investigated by a variety of solid 7Li and ³¹P NMR.The results of 7Li pulse gradient field-NMR（7Li PFG NMR）at different temperature,which can avoid grain boundary interference,show that the intrinsic self-diffusion coefficient（DLi⁺）of Li₁₀SnP₂S_12-xSe_x electrolytes firstly increases and then decreases with the increase of x,and has the highest diffusion coefficient(4.08 × 10^-12 m2/s)at x=5.This is consistent with the trend of ionic conductivity.Through ³¹P MAS NMR experiments,we obtain the basic law of Se’s preferred substitution and specific number of S for Sn（1）/P（1）S4 and P（2）S4 tetrahedrons in Li₁₀SnP₂S₁₂ structure.In contrast,conventional diffraction methods cannot provide information about this local structure.Through the spin lattice relaxation time（T1）of 7Li and ³¹P at variable temperature,we found that the substitution of Se can affect the activity of Li（4）ions in the ab plane by changing the local structure of Li₁₀SnP₂S_12-xSe_x electrolytes,especially the local structure around P（2）S4.The two-dimensional（2D）Li⁺conduction channel located in plane ab in the Li₁₀SnP₂S₁₂ structure with quasi-one-dimensional（along the c-axis）Li⁺diffusion path is "re-opened"（x=5）and "partially blocked"（x=7）,thus affecting the diffusion rate of Li⁺.Macroscopically,the ionic conductivity and diffusion coefficient firstly increase and then decrease with the increase of x.By studying the macroscopic ionic conductivity and the transport mechanism of Li⁺ in the microscopic structure of Li₁₀SnP₂S_12-xSe_x solid electrolyte materials,we have an in-depth understanding for the mechanism of action and structure-activity relationship between Se and vertex S substitution in Li₁₀SnP₂S₁₂,and realize the improvement of ionic conductivity.（2）The Se-doped Li₁₀SnP₂S_12-xSe_x（x=0,2,3.5,7）sulfide solid electrolytes not only improve the ionic conductivity of Li₁₀SnP₂S₁₂,but also significantly enhance the air stability and the interfacial stability against lithium metal.We studied the effect of Se doping amount on air stability and constructed a high stability interface layer of sulfide inorganic solid electrolyte on lithium metal anode.Li₁₀SnP₂S₇Se₅ and Li10SnP2S5Se7 with high Se content were studied via 7Li,6Li and ³¹P MAS NMR.The results show that Se doping can effectively enhances the structural stability of electrolyte and inhibits the formation of high resistance byproducts Li2S and Li3+xP,thus significantly improving the interface stability between Li₁₀SnP₂S₁₂ and lithium metal anode.Combined with the high ionic conductivity of Li₁₀SnP₂S₇Se₅ and the high interfacial stability of Li10SnP2S5Se7,we designed the same series of sulfide electrolyte layer（7-5-7Se）,which achieved high capacity charging-discharge and remarkable cycle stability performance in all-solid state batteries.（3）A novel sodium-poly（tartaric acid）borate salt（NaPTAB）with low cost and environmentally friendly was synthesized by the aqueous phase synthesis method.NaPTAB was combined with PVDF-HFP and swelled with PC solution to obtain the single sodium-ion conductor gel polymer electrolyte（NaPTAB-SGPE）.The NaPTABSGPE has satisfactory ionic conductivity up to 1.43 × 10-4 S/cm,wide electrochemical window as high as 4.8 V（vs.Na⁺/Na）and high sodium-ion transference number of 0.91 at 60℃.Except these pleasurable performances,the Na3V2（PO4）3/Na cells assembled with NaPTAB-SGPE present excellent charge-discharge performances and stable cycling capabilities at high temperature（60℃）.The structure of NaPTAB salt was characterized by 1H and 11B liquid NMR,and the interfacial stability of NaPTAB-SGPE membrane was studied by ex-situ 23Na MAS NMR.（4）The oxygen vacancy-rich SnO₂ nanoparticle composites with biomass nitrogen-doped carbon microspheres（SNC composite materials）were fabricated by hydrothermal methodand used as anode materials of sodium-ion and lithium-ion batteries.The structural transformation of SnO₂/C anode materials and the formation potential and composition of SEI on the electrode surface were investigated by ex-situ 23Na,19F and 119Sn MAS NMR and ex-situ EPR combined with ex-situ XRD technique.We propose a new sodium storage mechanism for SnO₂-based anode materials:It was found that the sodium oxide existed in the form of sodium peroxide（Na₂O₂）but not Na₂O during the conversion of SnO₂.Meanwhile,a new discharge mechanism（xNa⁺+SnO₂（tetragonal）+xe-→NaxSnO₂（amorphous）,SnO₂+ 2Na⁺+2e-→Sn+Na₂O₂（amorphous）,Sn+xNa⁺+xe-→NaxSn（0≤x≤3.75））based on the transformation of（O₂）n-species(O₂^3-→O₂^2-).In addition,we also studied the mechanism of SEI formation on the negative electrode surface at high potential（about 1.1V）and the main components of SEI are NaF and Na₂CO₃.